Electrolytic process for the manufacture of alpha-ketoglutarate esters

ABSTRACT

Continuous process for the production of a mixture of delta monoalkyl Alpha -ketoglutarate, dialkyl Alpha -ketoglutarate and the corresponding ketals by electrolysis of 2-furoic acid in acidic alcoholic solvent.

Elie States Patent [111 3,871,977 Scanio Mar. 18, 1975 ELECTROLYTICPROCESS FOR THE 3,574,072 4/1971 Louvar 204/72 MANUFACTURE OFALPHA-KETOGLUTARATE ESTERS Inventor:

US. Cl. 204/78, 204/59 R Int. Cl C07b 3/00, BOlk 1/00 Field of Search204/78, 72, 59 R References Cited UNITED STATES PATENTS 8/1955Clauson-Kaas et ul. 204/78 Primary ExaminerR. L. Andrews Attorney,Agent, or Firnz-Connolly and Hutz [57] ABSTRACT Continuous process forthe production of a mixture of 8-monoalkyl a-ketoglutarate, dialkyla-ketoglutarate and the corresponding ketals by electrolysis of 2-furoic acid in acidic alcoholic solvent.

6 Claims, No Drawings ELECTROLYTIC PROCESS FOR THE MANUFACTURE OFALPHA-KETOGLUTARATE ESTERS BACKGROUND OF THE INVENTION The presentinvention relates to a process for producing esters and ketals ofa-ketoglutaric acid. More particularly the present invention concerns aprocess of producing a mixture of a-ketoglutarate fi-monoesters,diesters and the ketals thereof by electrolysis of an acidifiedalcoholic solution of Z-furoic acid. 2,5-Dialkoxy-2,5-dihydrofuran-2-carboxylic acid and the corresponding alkylester which are obtained as byproducts in the same electrolysis reactionmay also be converted to a-ketoglutarate esters by subjecting theacidified alcoholic electrolysis product mixture to somewhat elevatedtemperatures followed by mild acid hydrolysis of ketals present, thusaffording a crude product which is composed almost entirely ofa-ketoglutarate esters.

While the a-ketoglutarate esters obtained by the process of thisinvention are themselves of use as chemical intermediates, they may befurther hydrolyzed to obtain a-ketoglutaric acid of excellent qualitysuitable for use as a food acidulant or as a food buffering agent whenused either as the acid or in the form of an alkali metal or alkalineearth salt thereof. a-Ketoglutaric acid also has utility as a rawmaterial for organic synthesis, e.g., in the manufacture of glutamicacid.

Prior art methods for either electrolytic or chemical oxidation ofZ-furoic acid or furfural have invariably resulted in decarboxylatedreaction products, except in those cases involving the oxidation offurfural to 2- furoic acid e.g., Organic Synthesis, Collective Vol IV,493 (1963). D. A. Deribas, et al., Russian Pat. No. 40,972 (1935); Chem.Abstr., 30,7127 (1936), obtained maleic acid upon electrolysis offurfural in sulfuric acid. N. Hellstrom, Svensk. Kern. Tidskr.,60,214-220 (1948); Chem. Abstr., 43l27le (1949) obtained l3-formylacrylic acid by electrolysis of either furfural or 2-furoic acid in 1%aqueous sulfuric acid using lead electrodes. M. Taniyama, Chem. Abstr.,53,4248a (1959), reported that succinic acid was prepared in good yieldby electrolyzing furfural using 5% sulfuric acid in the anodic cell andsulfuric acid in the cathodic cell. The intermediate maleic acid was notisolated but immediately reduced. Oxalic, tartaric, citric and malicacid byproducts were identified by a systematic separation method.

Both chemical and electrolytic methods for the preparation of2,5-dialkoxy-2,S-dihydrofurans were reviewed by N. Elming in Vol. II ofAdvances in Organic Chemistry, Methods and Results, R. A. Raphael, E. C.Taylor and H. Wynberg, Editors, New York, 1960, pp. 67l 15. No mentionis made of conversion of 2 -furoic acid to the corresponding 2,5-dialkoxy-2,5- dihydrofuran although the methyl and ethyl esters of2-furoic acid as well as 4-alkyl and 5-alkyl substituted 2-carbomethoxyfurans are reported to provide good yields of the corresponding2,5-dimethoxy-2,5- dihydrofurans.

M. Murakami et al., Proc. Japan Acad., 32135 (1956), Chem. Abstr. 50,l5504i (1956), reported that diethyl a-ketoglutarate can be obtained bytreatment of ethyl 2,5-diethoxy-2,5-dihydro-2-furoate with absoluteethanol containing dry hydrogen chloride. Electrochemical oxidation offurans was later reviewed by N.

L. Weinberg,'Chem. Revs., 68,449 (1968); no disclosure of the presentinvention is made therein or in a subsequent studies by S. Torii et al.,Bull, Chem. soc. Japan, 44,1079 (1971); 45,2783-87 (1972).

Other prior art disclosing electrolytic oxidation of 2- furoates but notknown to disclose the process of the present invention include US. Pat.Nos. 2,714,576, 2,801,252, 2,806,852 and Japan Pat. No. 23,727(l964),283 (1951).

SUMMARY While prior art methods of electrochemical oxidation of 2-furoicacid have invariably afforded decarboxylated products, the presentinvention discloses a novel process for the production of esters ofa-ketoglutaric acid by electrochemical oxidation of 2-furoic acid, inwhich the carboxyl groupremains intact in the major products. Moreparticularly the invention discloses a continuous process for producinga mixture of 8-monoesters and diesters of a-ketoglutaric and thecorresponding ketals thereof which comprises electrolyzing a solution of2-furoic acid in alcoholic solvent containing acid catalyst at aconcentration of about 0.01 to 1 mole per liter, wherein said alcoholicsolvent is an alkanol of from about one to five carbon atoms, andwherein said acid catalyst is a reaction inert acid of pK less thanabout 2.5, and continuing said electrolysis until a substantial amountof said mixture of esters and ketals of a-ketoglutaric acid is formed.The electrolyzed solution also contains 2,5-dialkoxy-2,5- dihydrofuran2-carboxylic acid and ester byproducts. The yield of oz-ketoglutarateesters and ketals thereof is enhanced by heating said electrolyzedsolution at about 50 to C. to substantially convert said byproducts toadditional amounts of said esters and ketals of a-ketoglutaric acid.Ketals present may 'then also be converted to a-ketoglutarate esters bysubjecting the reaction mixture to mild acid hydrolysis. Thea-ketoglutarate esters can be isolated by standard methods or hydrolyzedunder acidic or alkaline conditions to afford excellent yields of 0z-ketoglutaric acid.

DETAILED DESCRIPTION OF THE INVENTION The novel process of the inventioncan be employed to prepare excellent yields of a mixture of 8-monoalkyla-ketoglutarates (I) and the corresponding diesters (II) it iROOCCH2CH2CCOOH RoocoHicmcoooa ROOCCHZCH2CCOOH III OR R OR COOH COOR Theelectrolyzed solution, of course, also contains small amounts ofunreacted 2-furoic acid and alkyl 2- furoate ester in which the alkylgroupis derived from the alkanol solvent.

In each of the above structures R is the alkyl group derived from thealkanol employed as electrolysis medium, said alkanol being of fromabout one to five carbon atoms. This initial mixture can then be treatedfurther by heating the electrolysis products in the presence of an acidcatalyst at a concentration of about 0.2-2.0 moles/liter to convert thebyproduct 2,5- diall oxy-2,5-dihydrofuran 2-carboxylates (V,Vl) toadditional a-ketoglutarate esters. The products in the solution thusobtained are almost entirely compounds of the structures I, II, III, andIV. Subsequent mild acid hydrolysis may then be employed to selectivelyconvert ketals III and IV to fi-monoalkyl a-ketoglutarate (I) anddialkyl a-ketoglutarate (II). Alternatively, the mixture of compounds ofthe structures 1, II, III and IV obtained by heating the electrolysisproducts in the presence of acid catalyst at a concentration of about0.2-2.0 moles/liter, may be directly converted to a-ketoglutaric acid bystandard methods such as for example, mild acid hydrolysis to decomposethe ketals, followed by more vigorous hydrolysis under either acidic oralkaline conditions. The a-ketoglutaric acid may then be isolated andpurified, if necessary, by methods well known to those skilled in theart.

5 vert Wherein any and all acids, organic and inorganic,

having a pK,, less than about 2.5 may be used as catalysts in theprocess of the invention, it is preferred to use those acids which arealso inert under the condi-, tions of the electrolysis. Such acidshaving pK less than about 2.5 and which are also inert to theelectrolysis conditions include sulfuric, phosphoric, perchloric,p-toluenesulfonic, naphthalenesulfonic, benzenesulfonic, dichloroacetic,trichloroacetic and pyrophosphoric acids. Sulfuric, phosphoric andperchloric acids are preferred. However, sulfuric acid because of itslow cost and ease of availability is especially preferred. Any of theseacids may also be used for catalysis of the above-mentioned reactionssubsequent to electrolysis. While is is usually preferred to utilize thesame acid as that employed for the electrolysis, a different acidselected from those mentioned above as well as hydrochloric andhydrobromic acids may also be used as catalyst for these reactionssubsequent to the electrolysis.

The alkanol employed in the process of this invention serves both as asolvent and as a source of alkyl groups for formation of the esters,ketals, and ethers produced in the electrolysis. The alkanol employedmay be methanol, ethanol, isopropanol, tertiary butanol, 3- pentanol, orany of the alkanols of from about one to five carbon atoms. For reasonsof economy the preamount of a-ketoglutarate esters and ketals (I, II,III and IV) in the electrolyzed solution also tends to increase, but notin 21 directly proportional manner. At acid catalyst concentrationssubstantially greater than about 1 molar undesirable reactions takeplace, including one or more which generate decarboxylated products. Atacid catalyst concentrations substantially below 0.01 molar currentefficiency is significantly reduced and the reaction stops at thedihydrofuran (V and VI) with little or no production of the desiredproducts I, II, III and IV.

For the conversion of byproduct 2,5-dialkoxy-2,5-dihydrofuran-2-carboxylates, V and VI to esters and ketals ofaketoglutaric acid, acid catalyst concentrations in the range of about0.2 to 2 molar are utilized. At acid catalyst concentrationssubstantially lower than 0.2 molar the conversion is exceedingly slow.At substantially higher concentrations than 2.0 molar decompositionproducts are observed. The optimum concentration of acid catalyst forthis conversion is about 1 molar.

When acid catalyst concentrations of about 0.2 to 1 molar are employedin the electrolysis solution, the electrolyzed products may be directlytreated to conbyproduct 2,5-dialkoxy-2,5-dihydrofuran-2- carboxylates toa-ketoglutarates without further addition of acid.

The formation of the ketals III and IV from 2-furoic acid via thedihydrofurans V and VI does not require water as illustrated below usingmethanol as solvent. However, one mole of water is required for theconversion of the ketal group to the corresponding ketone group. Thus,when water is rigorously excluded from the electrolysis and subsequenttreatment at 50l00C. in 0.2-2M acid catalyst, the products obtained arealmost entirely ketals of the general structures III and IV. Whencommercial quality 2-furoic acid, alcoholic solvent and mineral acid areemployed small amounts of moisture ae normally contained therein.Accordingly, the products of the process of the invention containgreater amounts of S-monoalkyl oz-ketoglutarate (I) and dialkyla-ketoglutarate (II) and little or none of the ketals. While such alevel of moisture is not detrimental to the instant process, it ispreferred to limit the amount of water introduced so that it does notsubstantially exceed the theoretical amount of one mole per mole of2-furoic acid employed in the electrolysis.

While the aforementioned electrolysis process of this invention may becarried out in a batch operation, it is perferably conducted in a flowcell to allow continuous production of the primary electrolysisproducts. The flow cell consists of a cylindrical carbon anode centeredwithin a hollow metal cylinder which serves as cathode. The cell may bemounted in a chemically inert, non-conductive vessel. and suitable meansprovided for continuous flow of 2-furoic acid solution,

electrical connections to a power source and standard instruments forcontrol and measurement of the pertinent electrical parameters. Saidinstruments are well known to those skilled in the art. Metals whichhave been found to function adequately as cathode include nickel,platinum, palladium, silver and gold, although other metals and alloysmay also be used. Nickel is especially preferred as the cathodematerial.

The optimum flow rate of alcoholic solution through the cell isprimarily governed by the concentration of 2-furoic acid in the solutionbeing electrolyzed and the current employed. The theoretical currentrequired is about 54 ampere hours per mole of 2-furoic acid. Thus, theflow rate may be increased as the concentration of 2-furoic acid isdecreased at constant current; and, conversely, flow is decreased athigher concentrations of substrate. Since electrochemical yields areusually somewhat less than theoretical, greater quantities ofelectricity are ordinarily required to obtain an optimum yield of thedesired product. The electrolysis is preferably carried out at constantcurrent such that total electrical consumption is from about 50-100ampere hours per mole of 2-furoic acid. Cell voltage should bemaintained in the range of about 0.01 to 1.0 volt/cm of anode surface,and preferably about 0.02 to 0.2 volts/cm of anode surface.

The electrolysis of 2-furoic acid when carried out according to theprocess of the invention is only mildly exothermic. Ordinarily it may becarried out at ambient temperature without the necessity of cooling orheating. However, it is preferred to maintain the temperature at aboutto 35C. during the electrolysis.

After adjusting the acid catalyst concentration, if necessary, to within0.2 2 molar, the electrolyzed solution may be heated at temperatures ofabout 50 to 100C. to effect substantial conversion of 2,5-dialkoxy-2,5-dihydrofuran-2-carboxylate byproducts (V and VI) to the desireda-ketoglutarate esters and ketals, I, ll, Ill, and IV. The time requiredto effect substantially complete conversion will vary, of course, withthe temperature, acid catalyst concentration, and the nature of thealkyl group derived from the alkanol used as solvent in theelectrolysis; however, from about 2 to 24 hours is usually sufficient.As mentioned above, water is not required for this conversion and itshould be limited to a maximum of about one mole per mole of 2-furoicacid employed in the electrolysis. When perchloric acid is used, it ispreferred to carry out this conversion at temperatures near the low endof the range 50-100C. for reasons of safety.

The following examples are illustrative of the invention.

EXAMPLE I 1.0 gram of 2-furoic acid was dissolved in 65 ml. of methanolcontaining 0.10 ml. of 98% sulfuric acid. A portion of the solution wasintroduced into a 15 ml. flow cell set within a glass tube. The cellconsisted of a cylindrical carbon anode (6 mm. diam., 23.5 cm. long)centered within a hollow cylindrical nickel cathode (1.2 cm. diam. 23.5cm. long). The glass tube containing the cell was fitted at either endwith rubber stoppers with holes to accomodate'glass inletand outlettubes as well as electrode leads.

The remainder of the methanolic solution was placed in a reservoirleading to the inlet of a peristaltic pump. The pump outlet was, inturn, connected to the cell inlet by means of inert flexible tubing. Atambient temperature, 200 ma. current was passed through the solution at1.0 volt. After a few minutes pumping was started and the inlet andoutlet to the cell were opened 5 to accomodate fresh 2-furoic acidsolution. Flow was regulated so that the total-electrolysis time was 140minutes during which a total of 480 milliampere hours current was passedthrough the cell. The comsumption of Z-furoic acid was monitored bymeasuring the decrease in absorbance at 243 nm.

The combined effluent from the cell was neutralized with methanolicsodium methoxide and the solvent was evaporated at reduced pressure toafford a solid residue. The residue was taken up in ether, filtered toremove sodium sulfate and the filtrate evaporated to dryness. Theresidue was chromatographed on a silica gel column, eluting with ethylacetate. Four fractions, tabulated below, were obtained. They wereidentified by nuclear magnetic resonance spectroscopy and by comparisonwith independently prepared samples.

Fraction No. Structure Comments 1 A major component, (EH 0 O CH; bothcis and trans isomers present.

0 H C O OH plus Do. 3 0 01130 0 CH3 0 H C 0 0 CH:

2 CIIQOfJOHQOHZEfCOOH A major component.

3 5 3 U Trace. L C 0 0 CH3 4 Do. .0 l l 0 C O O H Gas-liquidchromatography of Fraction 2 revealed the presence of about 5% dimethyla-ketoglutarate and lesser amounts of 8-monomethyl a,a-dimethoxyglutarate and dimethyl a,a-dimethoxy glutarate in this fraction. The GLCwas carried out using a 5 ft. X A in. SE- column at 175C.; thermalconductivity detector temperature, 225C; Helium flow rate, ml/min.

EXAMPLE II The experiment was conducted under rigorously anhydrousconditions throughout.

A solution of 1.26g. of 2-furoic acid and 0.83g. of H SO in ml. ofmethanol was prepared and electrolyzed as described in Example I. Themethanol was removed in vacuo and the residue was dissolved in 10 ml. ofmethanol containing 0.5g. of H 80 to give a solution 1.4 molar in H 80The resulting solution was heated at reflux temperature for 24 hours,cooled and the methanol removed at reduced pressure.

The residual oil was chromatographed on a silica gel column, elutingwith benzene/ethyl acetate mixtures. Reduced amounts of the productsfound in Example I were obtained as well as a fraction containing thelarge amount (102g) of a new material. This material was found to be atleast 95% pure by gas-liquid chromatographic analysis on an SE-30 columnat 175C. It was identified by means of NMR and infrared spectra asdimethyl 2,2-dimethoxyglutarate.

EXAMPLE III To 90 ml. of 0.015 M methanolic sulfuric acid, 2.5g of2-furoic acid was added. The solution was electrolyzed under theconditions described in Example I. The electrolyzed solution was treatedwith 7.2g. of conc. H 80, and then heated to reflux for 2 hours, cooledto about 20C. and aqueous 5M NaOH was added in portions to effectcleavage of ketals and neutralization of the excess H 80 Afterfiltering, solvent was removed by distillation to afford 3.1g (ca. 90%of theory) of a mixture of d-monomethyl a-ketoglutarate and dimethylaketoglutarate.

EXAMPLE IV Seven grams of 2-furoic acid was dissolved in 250 ml. ofmethanol containing 0.37g. of H 50 The solution was electrolyzed over a6 hour period using the procedure and equipment described in Example I.The total current consumption was 3.4 ampere hours at 2.2 volts. Theaccumulated cell effluent was treated with an increment of H SO toadjust the concentration of H 80 to 1.0 molar. The resulting solutionwas heated to reflux for 2.5 hours, then cooled to room temperature.Sufficient water was added to hydrolyze the ketal, then an excess of anaqueous suspension of Ca(OI-I) was added, the mixture heated to refluxfor an additional 2 hours then filtered. The solid was suspended inwater (200 ml.), an excess ofH SO was added and the resulting CaSOremoved by filtration. Concentration of the aqueous filtrate afforded 8.lg (88% of theory) of a-ketoglutaric acid.

EXAMPLE V One gram of 2-furoic acid and 0.28g of 70% perchloric acid aredissolved in 25 ml. of absolute ethanol. The solution is electrolyzed asdescribed in Example I except that the cathode employed is a platinumfoil cylinder (1.2 cm. diam. 23.5 cm. long). The total current consumedamounts to 525 milliampere hours over 2.5 hours. The electrolyzedsolution is then treated with 044g. of perchloric acid andwarmed withstirring at 50C. for 5 hours. The solution is cooled to 10-15C. andaqueous 5M KOI-l added in portions to hydrolyze ketals and neutralizethe excess mineral acid. The solvent is then removed in vacuo on arotary evaporator and the residue filtered to afford a mixture ofS-monoethyl-and diethyl a-ketoglutarate.

EXAMPLE VI Two grams of 2-furoic acid is dissolved in 125 ml. ofn-propanol containing 12.3g of H SO The solution is electrolyzed asdescribed in Example V over 5.5 hours during which a total of 1.15ampere hours passes through the cell at a potential of 1.5 volts. Thesolution is then heated at 55C. for 4.5 hours, cooled to roomtemperature at treated with 10 ml. 10% aqueous NaOH and stirred for 30minutes to hydrolyze the ketals. Then additional base is added toneutralize the excess mineral acid. Following filtration to remove theprecipitated salt, propanol and moisture are removed by evaporation atreduced pressure to afford a mixture of 8-monopropyl-and dipropyla-ketoglutarate in good yield.

EXAMPLE vn 2-Furoic acid, 7.0g. is dissolved in 175 ml. of methanolcontaining 0.4g. 85% H PO The solution is electrolyzed as described inExample I, but maintaining the temperature at 35C. over a 4.5 hourperiod during which 3.7 ampere hours of current flowsthrough the cell at1.8 volts. The electrolyzed solution is treated with a mixture of 8.65g.85% H PO 1.4g. H 0 and 7.1g. P 0 to afford a methanolic solution that is1 molar in phosphoric acid. After refluxing 4 hours the solution iscooled to room temperature, 10 ml. of water added and the solutionallowed to stand overnight. An excess of aqueous Ca(OH) slurry is thenadded and the resulting mixture refluxed for 2 hours, cooled andfiltered. The filter cake is suspended in water and made strongly acidwith 85% H PO The insoluble calcium phosphate is removed by filtrationand the filtrate concentrated in vacuo to afford an excellent yield ofa-ketoglutaric acid.

EXAMPLE VIII In 125 ml. of methanol, 0.2g H SO and 3.5g. 2- furoic acidis dissolved and the solution electrolyzed by the procedure described inExample 1 except that the temperature is maintained at about 15C. Tengrams of sulfuric acid is added and the resulting solution heated atreflux for 2 hours. To the warm solution 50 ml. of water is added andreflux continued for an additional 4 hours. The excess acid is thenneutralized with aqueous base and evaporated to dryness at reducedpressure. The residue is triturated with acetone, filtered to removeinsoluble salts and the filtrate diluted with benzene and set aside tocrystallize. A good yield of oz-ketoglutaric acid of high quality isobtained.

EXAMPLE IX Example III is repeated using 3-pentanol in place of methanolas solvent. After electrolysis at 2030C, 7.2 grams of conc. H 50 isadded and the solution is heated at C. for 4 hours then cooled to roomtemperature. Aqueous 5M NaOH is added in portions to effect cleavage ofketals and then neutralization of the excess H After filtration, thesolvent is removed at reduced pressure to afford a good yield of amixture of 8-mono-3-pentyl a-ketoglutarate and the 3-pentyla-ketoglutarate diester.

What is claimed is:

1. A continuous process for producing a mixture of S-monoesters anddiesters of a-ketoglutaric acid and the corresponding ketals thereofwhich comprises electrolysis of a solution of 2-furoic acid in alcoholicsolvent containing acid catalyst at a concentration of from about 0.01to 1 mole per liter, wherein said alcoholic solvent is an alkanol offrom about one to five carbon atoms, and wherein said acid catalyst is areaction inert acid of pK less than about 2.5, and continuing saidelectrolysis until a substantial amount of said mixture of esters andketals of a-ketoglutaric acid is formed.

2. The process of claim 1 in which said acid catalyst is selected fromthe group consisting of sulfuric, phosphoric and perchloric acids.

3. The process of claim 1 in which said alcoholic solvent is methanoland said acid catalyst is sulfuric acid.

4. The process of claim 1 in which the electrolysis is carried out atabout l535C.

6. The process of claim 5 wherein additional acid having a pK less thanabout 2.5 is introduced to said solution prior to said heating to bringthe total acid catalyst concentration within the range of from about 0.2

to 2.0 moles per liter.

1. A CONTINUOUS PROCESS FOR PRODUCING A MIXTURE OF $ MONOESTERS ANDDIESTERS OF A-KETOGLUTARIC ACID AND THE CORRESPONDING KETALS THEREOFWHICH COMPRISES ELECTROLYSIS OF A SOLUTION OF 2-FUROIC ACID IN ALCOHOLICSOLVENT CONTAINING ACID CATALYST AT A CONCENTRATION OF FROM ABOUT 0.01TO 1 MOLE PERLITER, WHEREIN SAID ALCOHOLIC SOLVENT IS AN ALKANOL OF FROMABOUT ONE TO FIVE CARBON ATOMS, AND WHEREIN SAID ACID CATALYST IS AREACTION INERT ACID OF PKA LESS THAN ABOUT 2.5, AND CONTINUING SAIDELECTROLYSIS UNTIL A SUBSTANTIAL AMOUNT OF SAID MIXTURE OF ESTERS ANDKETALS OF A-KETOGLUTARIC ACID IS FORMED.
 2. The process of claim 1 inwhich said acid catalyst is selected from the group consisting ofsulfuric, phosphoric and perchloric acids.
 3. The process of claim 1 inwhich said alcoholic solvent is methanol and said acid catalyst issulfuric acid.
 4. The process of claim 1 in which the electrolysis iscarried out at about 15*-35*C.
 5. -dialkoxy-process of claim 1 whereinfollowing said electrolysis said solution is heated at from about 50* to100*C. to substantially convert 2,5-dialkoxy-0b2,5-dihydrofuran-2-carboxylate byproducts of said electrolysis to anadditional amount of said mixture of delta -monoesters and diesters ofAlpha -ketoglutaric acid and the corresponding ketals thereof.
 6. Theprocess of claim 5 wherein additional acid having a pKa less than about2.5 is introduced to said solution prior to said heating to bring thetotal acid catalyst concentration within the range of from about 0.2 to2.0 moles per liter.